The invention is related to a transmission system comprising a transmitter being coupled to at least one receiver via a transmission medium, the transmitter comprising frame assembling means for assembling frames comprising a control portion and a sequence of ATM cells, the transmitter further comprises validity indication inserting means for inserting an validity indication in the control portion regarding the validity of the sequence of ATM cells, the receiver comprising frame disassembling means for extracting the ATM cells from the frames if the validity indication indicates valid ATM cells.
The invention is also related to a transmitter, a receiver, a transmission method and a signal.
A transmission system according to the present invention is known from DAVIC 1.1 Specification, Part 8, Revision 3.0.
In the DAVIC specifications (Digital Audio Video Council), it is tried to standardise a digital enhanced broadcast chain. These standards cover the complete chain from the content provider via the service provider to the end user. Aspects covered by DAVIC are e.g. video coding, security, channel coding, modulation and frame structures.
In DAVIC it is proposed to use MPEG-2 transport stream multiplex packets, in which the 187 bytes payload part carry a frame comprising a control portion and a sequence of ATM cells. The control portion can carry several items such as the priority of a packet, an error flag, an indication of the first frame of a multiframe sequence. It can also carry an indication that the current sequence of ATM cells carried in the frame has to be discarded at the receiver because the ATM cells are introduced as stuff information to fill the transmission channel.
A problem of the known system is that it is not suitable for transporting data in other formats than ATM cells. Such formats can e.g. be STM, which requires fixed length data words having a repetition rate of 125 xcexcs, or variable length data formats such as Ethernet packets or IP (Internet Protocol) packets.
The object of the present invention is to provide a transmission system according to the preamble which allows the transport of various types of data formats, without loosing the possibility of transporting ATM packets as described above.
Therefor the transmission system according to the present invention is characterised in that the frame assembling means are also arranged for inserting data different from a sequence of ATM cells into the frame, in that the validity indication insertion means are arranged for introducing an identification of the data different from a sequence of ATM cells by a different value of the validity indicator, and in that the frame disassembling means are arranged for extracting the data different from a sequence of ATM cells from the frames, if the validity indicator indicates data different from a sequence of ATM cells.
The present invention is based on the idea that it is possible to use the validity indication which is used for indicating the validity of the present sequence of ATM cells to indicate other types of data formats. By doing so, the transport of ATM cells according to the prior art is not affected at all, but only the possibility to add other types of data formats is created. In the DAVIC standard the control portion comprises 5 bits xe2x80x9c11110xe2x80x9d indicating the validity of the ATM cells. Consequently other values of these 5 bits can be used for indicating other data formats.
An embodiment of the present invention is characterised in that the data different from a sequence of ATM cells comprises packets with an identification portion indicating of at least one property of said packets.
By specifying at least one property of a cell in an identification portion, it is obtained that such cells easily can be identified and processed accordingly.
A further embodiment of the invention is characterised in that the at least one property comprises the length of the packet and in that the frame comprises a data portion indicating the position of the beginning of a new packet in the frame.
By specifying the length of each packet in the frame, and by specifying the beginning of one packet in the frame, the position of each packet in the frame can easily be determined. This allows an easy extraction of the packets from the frame.
A further embodiment of the invention is characterised in that the length of the packet is an integer multiple of an elementary data unit larger than one byte.
By introducing these elementary data units, the length of the packets can have only a restricted number of values, reducing the number of bits required to encode the length of the packets.